A photometer is the key of a automatic biochemical analyzer, which is required to have high accuracy and wide linear range. In a photometric measurement system, the stray light is one of the main factors that influence accuracy and linearity of absorbance, so the stray light must be reduced to an extent small enough. In a grating dispersing optical system, the zero order spectrum corresponds to the ray whose order of spectrum is zero Since the zero order spectrum is non-dispersive, that is, it integrates the zero order spectral energy of all wavelengths the zero order spectral energy takes up a relatively large proportion of the total incident energy. This portion of ray whose order of spectrum is zero is reflected and scattered in the apparatus to form the stray light, therefore forming an interfering background at the position of detecting a spectral signal and reducing the signal-to-noise ratio and analytical sensitivity of the apparatus.
In order to eliminate the stray light caused by the reflection and scattering of the zero order spectrum, the following solutions are generally employed: 1) utilizing a concave grating to minimize the number of optical elements ill a light splitting chamber, thus reducing the stray light generated by scattering at the optical surface; 2) blackening the components in the light splitting chamber by spraying a black extinction paint over the vessel surface that generates the stray light or coating the vessel surface with a black cotton flannel. However, these two methods can not suppress sufficiently the stray light generated by the zero order spectrum.